Triphenylmethane (TPM) dyes have been historically produced in industry using chemical oxidizing agents like sodium dichromate, lead dioxide and manganese dioxide. While such processes are well known and are practiced in industry, the use of such chemical oxidizing agents can present environmental and waste disposal issues which, in order to be handled effectively, increase the costs for the practice of the process. One of the main advantages of an electrochemical process is that such processes use electricity to drive the reaction rather than using reactive chemicals. Electrochemistry eliminates the hazardous handling of these chemicals, the cost of buying them, and the final disposal costs of the spent metal byproducts. This an excellent example of “green chemistry” which strives to develop environmentally benign processes.
It has been shown in the literature (U.S. Pat. No. 4,775,451, Habermann, et al., issued Oct. 4, 1988) that TPM dyes can be formed by the electrochemical oxidation of their corresponding leuco dyes using expensive electrodes coated with precious metal catalysts. The anodes utilized in the Habermann, et al. process are made from metals, such as titanium, tantalum or niobium, surface doped with metal oxides. The patent teaches that the surface of the electrodes should not be roughened (see column, lines 38–40, and column 4, lines 29–34). The patent also teaches that the presence of low levels of a C2–C4 alcohol, urea, a urea derivative, or a mixture of those materials in the solution being oxidized will produce high yields of high purity TPM colorants.
The use of graphite carbon felt anodes has been disclosed for several types of electrosynthesis reactions. See, for example, Peacock, et al., Tetrahedron Letters (2000), 41(46): 8995–8998. However, such electrodes have not heretofore been taught for use in oxidation reactions for the formation of colorants, and triphenylmethane colorants in particular.
The present invention provides an electrochemical oxidation process for the preparation of colorants, particularly triphenylmethane colorants, using graphite carbon felt electrodes. Surprisingly it has been found that the use of such electrodes in this process provides reaction rates which can be as much as ten times faster than the rates found using precious metal flat electrodes, and can provide process yields as much as ten or fifteen percent greater than current plant processes utilizing chemical oxidizing agents such as manganese dioxide. These excellent results are accomplished using electrodes which are considerably less expensive than the precious metal electrodes described in U.S. Pat. No. 4,775,451. Additionally, the improved reaction yields and reduced cycle times allow the oxidation to take place in smaller or fewer electrochemical cells thereby greatly reducing capital costs for colorant production. Finally, the present invention allows for the synthesis of colorants, particularly TPM colorants, without requiring the use of chemical oxidizing agents, thereby eliminating any environmental or safety considerations regarding the use or disposal of such agents.